A high temperature mass spectrometric method is used for the identification of vapour species over the CaO-Al₂O₃-SiO₂, CaO-TiO₂-SiO₂ and BaO-TiO₂-SiO₂ systems in the concentration range where the mole ratio of CaO(BaO):SiO ₂ is equal to unity at 1800-1950 K. The vaporisation processes are similar to those in the individual oxides contained in the systems studied. Activities of components in these systems were obtained by the ion current comparison method. Negative deviations from the ideality were observed in the CaO-Al₂O₃-SiO₂ systems and changes of the sign from the ideal behaviour are found in the CaO-TiO₂-SiO₂ and BaO-TiO₂-SiO₂ systems. Thermodynamic properties of the systems under consideration are also calculated using the generalised lattice theory of associated solutions. Suitable agreement was shown between experimental and calculated results. The numbers of the various types of bonds in these melts were calculated using the same theory. The dependence of the number of Ca-O-Ca, Al-O-Al, Si-O-Si, Ti-O-Ti, Ba-O-Ba, Ca-O-Al, Ca-O-Si, Ca-O-Ti, Ba-O-Si, Ba-O-Ti, Al-O-Si, Ti-O-Si bonds as a function of the concentration of oxides in the CaO-Al₂O₃-SiO₂, CaO-TiO₂-SiO₂ and BaO-TiO₂-SiO₂ systems is in good agreement with the deviations from ideality of their thermodynamic functions.